The invention relates to a protection device for low-voltage power supply systems having a measurement device and a downstream evaluation device.
Electrical systems are stressed electro dynamically and thermally by short circuits. This stress is influenced both by the time which passes before the short circuit is identified (so-called short-circuit identification time) and by the time which is required to disconnect the short circuit (so-called disconnection time).
As is known, electronic and microprocessor-controlled evaluation circuits are used to identify overload states, but these essentially simulate the function of a bimetallic strip. In simple devices, magnetic tripping coils and bimetallic strips are used, which do not offer optimum protection. Virtually without exception, short circuits are disconnected by means of mechanical switches, with the contact separation normally starting only after a specific reaction time, and the arc which is formed resulting in a voltage which opposes the power supply system voltage. This arc voltage limits the short-circuit current and, finally, causes it to be extinguished. Switches based on conventional semiconductors are used only in exceptional cases owing to the high forward losses during normal operation.
An electrical power distribution system having automatically operating protection devices is known from EP 0 838 887 A2, in which a unit for early short-circuit identification is provided. This unit takes account of the instantaneous value of the current on the one hand, and its rate of change on the other hand. In the process, conventional circuit breakers are actuated by the unit for early short-circuit identification.
The object of the invention is to produce a protection device which identifies anomalous operating states very quickly in order to provide adequate protection against overloading and short-circuits for switching devices in low-voltage power supply systems.
According to the invention, the object relating to a protection device of the type mentioned initially is achieved in a first embodiment of the invention by the features of patent claim 1, with the dependent patent claim 2 including a specific development. In a second embodiment of the invention, the object is achieved by patent claim 3, with patent claims 4 and 5 including specific developments. Developments which apply to both alternatives are specified in the other dependent claims.
In both alternatives of the invention, the evaluation device is a unit for early short-circuit identification, which actuates a semiconductor switch, with this resulting in the faulty branch being disconnected from the power supply system. The semiconductor switch is preferably produced based on silicon carbide (SiC), as is mentioned, for example, in the case of an electronic branch switching device according to DE 196 12 216 A1. A switch such as this is preferably in the form of a so-called JFET.
In the invention, the combination of a unit, which is known per se, for early short-circuit identification with known semiconductor switches unexpectedly results in advantageous short-circuit disconnection characteristics, which can be explained by a combinational effect. This combinational effect was not regarded as being feasible, in the specialist world. Alternatively, the unit for early short-circuit identification operates on the basis of the product of the current and the current rate of change, or on the basis of so-called Tolerant Locus Curves (TLC). Especially in the case of the latter method, locus curves which take account of different power factors and initial currents are used as tripping criteria in a locus curve representation of the current and current rate of change.
The invention essentially combines device which have already been proposed in the prior art, in a novel manner. Switches based on silicon carbide are also known from recent publications. Early short-circuit identification using the current and current rate of change represents conventional prior art, while a method which operates on the principle of so-called tolerant locus curves for early short-circuit identification (TLC method) has been proposed in the prior, not previously published, EP 0 993 695 A2. In particular, those units, which operate using the TLC method, may be designed on an analog, digital or hybrid basis. The essential feature in all cases is that the switching work which needs to be carried out in the semiconductor switch is reduced by rapid short-circuit identification. This leads to smaller semiconductor volumes and thus, in particular, to smaller switches.